Ag alloys, the primary solute metal of which is Sn of a comparatively large amount, such as more than 4.5 weight %, can be completely internal-oxidized in Ag matrices with the help of In and/or Bi. Such Ag-Sn system alloys which contain Sn of more than 4.5 weight % to 10 weight % and In of 0.1-5 weight % and/or Bi of 0.01-5 weight % and which have been internally oxidized, are widely used today as electrical contacts for various electric and electronic appliances. Ag-SnO system alloy electrical contact materials of this kind are disclosed in publications such as U.S. Pat. No. 3,933,485, No. 3,933,486, and No. 4,243,413.
The aforementioned kind of internally oxidized Ag-SnO system alloys are one of the best materials of today for making electrical contact materials having excellent physical and electrical characteristics. However, as they contain a comparatively large amount of Sn, their oxidized solute metals including SnO tend, especially when they have comparatively large dimensions, to segregate too much at outer surface areas, and deplete inner areas, as a result of internal oxidation. Such segregation of oxides within Ag matrices brings about unstableness of electrical and physical characteristics, especially the contact resistances of the materials.
On the other hand, electrical contact materials which are made from powders of Ag and metallic oxides by a powder metallurgical method, can avoid the aforementioned kind of segregation. Nevertheless, those made from powders can hardly compete with those materials which have been alloyed and internally oxidized, because the former are inherently coarse in structure and wear too rapidly even under a normal operating condition.
In view of the above, this invention is to provide a method for preparing internally oxidized Ag-SnO system alloy contact materials having substantially no segregation of metallic oxides therein and having dense structures.